The long-term objectives of this study are to examine the neural and neurochemical regulation of respiratory rhythm and respiratory motor output during development in anuran amphibians. Normal maturation of the respiratory central pattern generator (CPG) and motoneuronal output is a necessary condition for life. The ontogenetic transition from an aquatic to a terrestrial habitat in amphibians (metamorphosis) is accompanied by major maturational changes of the respiratory system. Despite the profound changes that accompany metamorphosis, very little is known concerning how central respiratory rhythm and motoneuronal output to respiratory pump muscles are regulated during development. Larval (tadpole) and adult bullfrog (Rana catesbeiana) will be used to test the following specific aims related to the development and regulation of central respiratory rhythm and motor output: 1) Development of respiratory rhythmogenesis shifts from a 'pacemaker'-driven CPG in larval amphibians to a network-driven CPG in adults, and this is accompanied by maturation of inhibitory (glycine/GABA) and excitatory (nitric oxide-cGMP) neuromodulatory systems; 2) Hypoglossal motoneurons (XII MNs) undergo cellular and synaptic maturation that facilitate the regulation of 'pump' muscles for lung ventilation in the developing amphibian. Isolated in vitro brainstem preparations that generate a spontaneous respiratory-related motor output from larval (pre-metamorphic and post-metamorphic) and adult animals will be used to carry out the specific aims. Whole nerve and single unit extracellular recordings from respiratory-related neurons, bath application and microinjection of excitatory and inhibitory neurochemicals into discrete brain locations, will be used to test a number of hypotheses related to the specific aims. The proposed research should provide greater insight into the role of development in the neural and neurochemical regulation of ventilation in vertebrates. The results of this project may provide a greater understanding of developmental mechanisms that may contribute to abnormal breathing states in mammals such as Rett Syndrome, SIDS and premature apneas of central origin.